Factsheet: Coastal and freshwater recreation monitoring

Submitted By

LAWA Admin

Author

LAWA

Published Date

12 FEB 2018

During the swimming season (around November/December to February/March), regional, unitary, district and city councils and public health agencies assess the health risks from faecal (‘poo’) contamination at swimming spots. At some river and lake sites, potentially toxic algae is also regularly monitored.

LAWA displays the results of the recreational monitoring. We check for new data regularly throughout the day so you have the most up to date information on the water quality at swim sites throughout Aotearoa New Zealand.

Why can water cause illness?

When contaminated by human or animal faeces (‘poo’), water can contain disease-causing bacteria, viruses and protozoa (such as salmonella, campylobacter or giardia). These organisms can present a health risk in water that is used for recreational activities, such as swimming. The most common illness is gastroenteritis but respiratory illness and ear and skin infections may also occur.

The quality of water for swimming is determined by measuring ‘faecal indicator bacteria’ (enterococci in coastal waters andE. coli in rivers and lakes) which indicate the levels of disease causing organisms in the water. See this factsheet on faecal indicator bacteria to find out more.

Potentially toxc algae (cyanobacteria) can become a problem in some lakes and rivers when there are large amounts present (known as toxic algal blooms). Some cyanobacterial species are known to produce poison or toxins, but they are not toxic all the time. See this factsheet on potentially toxic algae for more information.

What does the summer recreational season monitoring information tell us?

For most sites, LAWA shows the recent water quality result, and the results from the last 3 years of sampling which is used to determine the overall bacterial risk grade. Both types of information can be used as a guide for you to decide where it is suitable to swim.

Weekly sampling & predicted water quality results

Faecal indicator bacteria

Most councils monitor the water quality weekly over the summer months to inform the public of any immediate risk from contact with the water. The weekly sample result is a recent snapshot in time - it tells you what the water quality was like at the time of sampling. It’s important to remember that water quality can change over time, especially after rain. That is why we recommend that even for sites with generally good water quality, that you avoid swimming 2 - 3 days after heavy rainfall as urban or agricultural run-off can affect bathing water quality.

In the Auckland region, water quality is predicted using modelled data. The predicted water quality results are regularly updated to give a 'real-time' result. The modelling takes environmental conditions (e.g. rain, tides, stormwater overflows etc.) into account when determining the water quality result at these sites.

When water quality falls in the ‘surveillance’ category this indicates that the risk of illness from swimming is low. If water quality falls into the ‘alert’ category, this indicates an increased risk of illness from swimming, but still within an acceptable range. However, if water quality enters the ‘action’ category, then the water poses an unacceptable health risk from swimming.

Note that under the MFE/MOH 2003 guidelines, health warnings for coastal waters are only issued after two consecutive samples (within 24 hours) exceed the ‘action’ level.

Potentially toxic algae

The risk to people and animals from potentially toxic algae is determined by measuring the proportion of the river bed covered by cyanobacteria as well as the amount of cyanobacteria mats washed up on the river’s edge, or the amount of cyanobacteria suspended in the water column in lakes or slow flowing waterways.

Not all recreational sites are monitored for potentially toxic algae. As it can be harmful, even in small amounts, we recommend you know what it looks like, and avoid contact when you see it.

Overall bacterial risk

The overall bacterial risk is a guide to give a general picture of water quality by determining a measure of health risk at a site. Updated annually, it is calculated from bacteria data (E. coli for freshwater or enterococci for coastal waters) collected over the last three years. This risk indicator doesn't include potentially toxic algal data.

The overall bacterial risk presented on LAWA is based on the 95th percentile of routine sample results from the last three years (re-tests are excluded from analysis). In simple terms, if a site was calculated to have a 95th percentile of 200, then this means that 95 out of 100 times that this site was monitored, that the results were at or below 200 E. coli / 100 mL for freshwater sites, or 200 enterococci/100 mL for coastal beaches.

A site must have at least 30 routine sample results, with at least 10 results in each year, for an overall bacterial risk to be calculated. LAWA has chosen to use three years of sampling so that weather conditions over several summers are taken into account.

LAWA uses the routine samples only (not additional sampling as a result of retests which may be taken when there are elevated results). If more than one routine sample is taken at a site in any week, then LAWA uses the first sample result to calculate the overall bacterial risk.

Wet weather can cause bacteria to flow from land into rivers and streams. Therefore wet weather can influence bacteria counts, and lead to higher risks being assigned after wet summers. Some councils factor the influence of rainfall into their monitoring results by limiting sampling to dry weather conditions when people are more likely to be swimming. Other councils remove rainfall-related bacteria results prior to calculating 95th percentile. Because there is not currently a standardised national method for adjusting rainfall-related sample data, no adjustments have been made to any bacteria data presented on LAWA (all routine sample results regardless of weather conditions, have been used in the risk calculations).

FAQ

What is the impact of rainfall?

Water quality at many river and beach swimming spots is affected in wet weather as a result of urban or rural runoff. In urban areas rainwater collected from roofs, roads, car parks and other surfaces is piped directly into rivers, streams and the coast. During its travels, this storm water picks up sediment, rubbish, contaminants, and dog and bird droppings. Sewer overflows can also occur in urban areas during wet weather. In rural areas, excess rainwater flows over the land and into nearby streams and rivers, picking up manure and other contaminants along the way. At some river, lake and coastal sites, heavy rain and wind can churn up sediments from the bottom of the waterway or sea, releasing pathogens in the sediments back into the water.

How can the weekly monitoring result be acceptable when the overall bacterial risk is high ('unsuitable for swimming')?

Sometimes a site can have an overall bacterial risk that is high but the most recent samples indicate that that water quality is within the acceptable range for swimming. This is because the overall bacterial risk is calculated using results over three years and, particularly for rivers, may have been impacted by a wet season causing high flows and bacteria contamination from run-off.

If the most recent weekly result is within the acceptable limits and there hasn’t been heavy rainfall since the sample date, it is likely that the water quality will still be safe for swimming. You can also view the history of the monitoring in the 'why this status' section to get an idea of what the water quality has been like in recent weeks and over the last three years.

What other risks are not accounted for?

Swimming at your favourite beach, river or water hole presents additional safety risks. At some river and lake sites the weekly testing does not measure the presence of potentially toxic algae (cyanobacteria) which may be harmful if ingested, stinging jellyfish, high water flow and strong tidal currents. Furthermore there are times when other contaminants that are not monitored can make bathing waters unsuitable for swimming. Please be aware of such risks when swimming.